The proposed investigations are comprised of three major parts: 1) Studies of Mammalian Erythropoiesis in Tissue Culture. Techniques of cell separation will be utilized to isolate a population of erythroid procursors from sources such as bone marrow or spleen. Different methods and conitions of culture will be evaluated with the goal of permitting these erythroid precursors to proliferate and mature in phased fashion in vitro. Such systems of culture would then be used for investigation of factors regulating the development of erythroid cells derived from animals and human subjects with physiologic or pathologic erythropoiesis (e.g., erythroleukemia). 2) Studies of Hemopoietic Cell Proliferation in Diffusion Chambers. Tissues from experimental animals and human subjects will be cultured in Millipore diffusion chambers implanted in the peritoneal cavities of mice with the aim of delineating factors that regulate the proliferation of hemopoietic cells, particularly those of the granulocytic series, and of elucidating pathophysiologic mechanisms in certain hematologic diseases (e.g., acute myelocytic leukemia). Particular attention will be given to the role of short-range regulators and microenvironmental factors in the growth and differentiation of blood cell precursors. 3) Isolation and Radioimmunoassay of Renal Erythropoietic Factor (REF or Erythrogenin). REF is a material produced by the kidney which interacts with a substrate in plasma to produce biologically active erythropoietin. Techniques of protein isolation will be used to purify REF and thereby permit is chemical characterization. Purified material would also be utilized for the developmnt of a radioimmunoassay, for investigation of the role of REF in regulating erythropoiesis in experimental animals and human subjects with alterations in red cell production.